You login as root, and then you just enter by the prompt "date MMDDHHMM". For instance, if I wanted to change the date to six hours ahead of my current time - which happens to be EST, although (or rather, since) I'm in Chile - I'd write "date 08020840". See?
Edit: Ah, to change the year also, it seems that you just append the year at the end of the date string.